Windows XP Service Pack 4 Unofficial is a cumulative update rollup for Windows XP (x86) English as well as security enhancements not addressed by Microsoft. 3.1b is expected to be the final release.
It can be applied to a live Windows XP system which has SP1, at the minimum, installed or it can be slipstreamed (integrated) in any Windows XP installation media.
It includes updates for most Windows XP components, including MCE and Tablet PC. Request-only hotfixes have been included. Currently, Microsoft.NET Frameworks 4.0, 3.5, 1.1 and 1.0 (Tablet PC only) are included in the Windows XP SP4 package. Also, all post-eol updates for. NET Framework 1.1, 3.5 and 4.0 until January 2016 have been included. The .NET 1.x, 3.5 versions are automatically installed/updated in live installations. Only Microsoft.NET Framework 3.5 is enabled by default in an integrated installation. Microsoft .NET Framework 1.1 will always be installed if Media Center Edition is present.
If you download the WindowsXP-USP4-v3.1b-x86-ENU.exe you can also scan Windows Update for POSReady updates because the POSReady trick has been included.
Windows XP SP4 security enhancements: Windows XP SP4 includes registry updates to enhance security provided by Stefan Kanthak, a security expert. These security enhancements address issues not fixed by Microsoft updates.
Should make the Chinese happy, they're the major pirates users of XP.